1. Field of the Invention
This invention relates to U.V. stabilized PVC articles, and, more particularly to an improved process of making surface modified, U.V. stabilized PVC articles having a high quality surface appearance.
2. Description of the Prior Art
Photodegradation of many types of solid polymeric articles by U.V. light has been a continual problem confronting the industry. Such U.V. degradation causes discoloration, embrittlement and deterioration of the article. Accordingly, U.V. absorber compounds have been incorporated into such polymeric articles in an attempt to provide effective stabilization to U.V. light.
However, since photodegradation is known to be a surface phenomenon, such incorporation of U.V. stabilizers into the polymeric material prior to shaping into solid form has been largely uneconomical and inefficient because the stabilizer is present substantially in the interior of the shaped article, where it does not function effectively to prevent surface photodegradation.
Accordingly, surface treatments have been proposed to provide more effective U.V. protection for such resins. These treatments are based on the ability of various organic solvents to swell such resins as polyvinyl chloride. This swelling property enables the ultraviolet light stabilizer material to be directed principally into the surface region of the resin body. However, none of these surface treatments has been very successful commercially. Another surface method comprises laminating the surface of the polymer with a layer of a film-forming material containing the U.V. stabilizer. This lamination approach, unfortunately, suffers from the problems of incompatibilities between polymer, U.V. stabilizer, and the film-forming material and its carrier components. Furthermore, the laminated layer and the surface of the polymer do not adhere well, and delamination upon shock or impact is a serious problem.
The swelling method is described by Katz, et al. in two articles appearing in the "Proceedings of the A.C.S Div. of Org. Coatings and Plastics" 36, (1) p. 202-205 (1976), entitled "Ultraviolet Protection of Transparent PVC by Diffusion Coatings" and in Soc. Plast. Eng., Technical Papers (1976) 22, p. 511-512. Katz uses a dip method to impregnate a transparent PVC sheet to a given depth with a nonmigrating ultraviolet light absorber. The dip technique comprises swelling the polymer with a carrier solvent, infusing the stabilizer from a solution thereof with the carrier, and, thereafter heating the treated polymer in air at an elevated temperature to drive the stabilizer below the surface of the polymer and to remove residual solvent. Under these process conditions, however, only a small amount of stabilizer is absorbed into the surface region, and residual stabilizer can remain on the treated surface.
Jochanan, in Israeli Pat. No. 39037, published Nov. 10, 1975, entitled "U.V.-and Oxidation-Proof Products from Organic Plastics and their Manufacture", describes various methods of applying a solution or suspension of an ultraviolet light absorber to the surface of a plastic sheet and air drying to remove solvent. The methods disclosed by Jochanan include immersion, spraying, brushing, rollerprinting and curtain-coating. However, with such methods, a large amount of residual stabilizer and solvent is left on the surface of the article, whereupon the modified surface is observed to be streaky, hazy, and has runs or pockmarks thereon.
Humphrey, in U.S. Pat. No. 4,146,658, describes a process for producing an ultraviolet light stabilized polycarbonate article by dipping the polycarbonate sheet into a impregnating solution consisting of a selected combination of ultraviolet absorber and solvent and evaporating the solvent.
Ching, in U.S. Pat. No. 4,349,607, describes the incorporation of certain U.V. stabilizers into thermoplastic resins by dipping, spraying and brushing followed by heating at 70.degree. C.
Amborski, in U.S. Pat. No. 3,043,709, discloses an organic polymeric article article having an ultraviolet light absorber compound incorporated adjacent to the surface of the article. The article is treated by several methods, including dipping-coating a polyvinyl chloride film in a stabilizer solution, and heating in air at 130.degree. C. to drive the absorber below the surface of the film and to remove the solvent. These processes, however, are disadvantageous because they form an article in which considerable residual absorber is present on both sides of the film, which affects its surface perfection and appearance.
Fanning, in U.S. Pat. No. 3,297,462, discloses a process for rendering polymeric films resistant to the deteriorating effects of sunlight by immersing them in a solution of a benzophenone stabilizer in a solvent mixture of diethylene glycol and glycerol, and then washing successively in diethylene glycol, at elevated temperatures, and then water. The polymers suitable for such treatment, however, must have a zero strength temperature of at least 200.degree. C., which excludes many useful polymers, including polyvinyl chloride.
Solvay and Cie, in Belgium Pat. No. 612,206, discloses a process for surface treating rigid polyvinyl chloride objects. The method comprises immersing the object in a swelling solvent containing a light stabilizer and then evaporating the solvent in air. This procedure, however, is ineffective because some stabilizer remains on the treated surface of the object, which gives a poor physical appearance, and, furthermore environmental hazards are created during removal of the solvent in air.
Cohnen, in U.S. Pat. No. 3,892,889, describes a method of applying a solution of a U.V. stabilizer to polycarbonate resin sheets and drying in air.
Chilton, in U.S. Pat. No. 4,323,597, teaches a flow-coating process for polycarbonate articles. In this process, the surface of the polycarbonate is heated and a liquid stabilizer composition, which is non-aggressive towards but wets the polycarbonate surface, is flowed over the heated surface. There is no indication in this patent, however, that the method is applicable to polyvinyl chloride.
Bristol, in U.S. Pat. No. 3,519,462, describes the treatment of heated polyvinyl chloride and other polymers with a solution of a U.V. stabilizer in a diol or triol. These solvents, however, are not efficient swelling agents for PVC. Furthermore, there is no indication therein of a suitable method for removing excess stabilizer solution present on the treated surface.
Lempkowicz et al, in U.S. 4,126,660 describes a process for the manufacture of oriented sheets of PVC which are surface modified with U.V. stabilizer by applying a solution of the stabilizer in a defined mixture of high boiling swellable solvents, at elevated temperatures, and thereafter evaporating the solvent from the treated surface.
In summary, these and other processes have not provided useful U.V. stabilized articles, particularly polyvinyl chloride resin articles, for example, for residential siding, which use requires particularly effective protection from the weathering effects of sunlight, and an appearance and degree of surface perfection which is acceptable in commercial use.
There is a present need to provide a method for effectively modifying the exposed surface of a PVC sheet, such as residential siding, with a U.V. stabilizer, such that the modified surface of the sheet is substantially free of residual stabilizer and solvent material.
It is an object of this invention to provide a method of making a surface modified, U.V. stabilized PVC resin sheet article wherein the surface region is modified with a U.V. stabilizer and the treated surface is substantially free of residual stabilizer and solvent material.